1. Field of the Invention
The present invention relates to cam grinding machines and cam grinding methods. More particularly, the present invention relates to a grinding machine and a grinding method for a composite cam in which two cams having different cam lifts and different phase angles are disposed to adjoin each other in the axial direction.
2. Description of the Related Art
Intake and exhaust processes to and from a cylinder of an internal combustion engine are performed by a valve opening operation. The valve opening operation is performed by operation of a rotary cam.
In order to improve output of the internal combustion engine etc., different valve opening control processes are used for the valve opening operation depending on whether the engine speed is high or low.
In one of such control methods, a first cam for low speeds and a second cam for high speeds are provided as a cam that operates a valve, and valve opening control is performed by selecting the first cam and the second cam as appropriate according to the engine speed. In this case, selective switching between the first cam and the second cam is carried out as a tappet of the valve moves relative to the first cam and the second cam in the axial direction while in contact with the first cam and the second cam.
FIGS. 21 to 23 are schematic views showing the positional relationship between a first cam 112 for low speeds and a second cam 114 for high speeds. As can also be seen from these figures, the first cam 112 for low speeds typically has low maximum lift height, and the maximum lift height of the second cam 114 for high speeds is typically greater than that of the first cam 112. The phase angles of the first and second cams 112, 114 are such that the phase of the second cam 114 for high speeds is ahead of that of the first cam 112 for low speeds with respect to their rotational direction (direction shown by arrow in FIG. 21), namely such that the valve opening operation is performed earlier by the second cam 114 than by the first cam 112. As shown in FIG. 21, the first cam 112 for low speeds and the second cam 114 for high speeds are positioned such that the cam profile in the lift height direction of the second cam 114 and the cam profile in the lift height direction of the first cam 112 are shifted from each other in the angular direction.
As shown in FIGS. 22 and 23, the first cam 112 for low speeds and the second cam 114 for high speeds are disposed so as to adjoin each other in the axial direction. Namely, the first and second cams 112, 114 are provided as a composite cam 110. In this case, both the first cam 112 for low speeds and the second cam 114 for high speeds are formed such that a base circle portion other than the cam profile that changes in the lift height direction according to the angle has a fixed radius r from the camshaft center. A certain angle range in which the base circle portions of the first and second cams 112, 114 overlap each other is a common base circle portion C. In the range of this common base circle portion C, the tappet moves relative to the first and second cams 112, 114 while in contact therewith to switch between the first and second cams 112, 114 as described above.
Grinding of the composite cam 110 comprised of the first cam 112 for low speeds and the second cam 114 for high speeds is usually performed with a cam grinding machine using a grinding wheel T (see FIGS. 22 and 23). Grinding of the composite cam 110 is implemented by performing plunge grinding of one of the first and second cams 112, 114 and then performing plunge grinding of the other.
For example, FIGS. 22 and 23 show the case where grinding of the first cam 112 for low speeds is first performed and then grinding of the second cam 114 for high speeds is performed. In this case, grinding of the first cam 112 is performed with the grinding wheel T based on preset cam lift data of the first cam 112 for low speeds. Thereafter, the grinding wheel T is moved to the position corresponding to the second cam 114 for high speeds, and grinding of the second cam 114 is performed with the grinding wheel T based on preset cam lift data of the second cam 114 for high speeds. Grinding of the composite cam 110 is thus performed with the cam grinding machine. See, e.g., German Patent No. DE 10333916 B4 and Japanese Patent Application Publication No. H04-13560 (JP H04-13560 A).
As shown in FIG. 23, in the above grinding of the composite cam 110 with the cam grinding machine using the grinding wheel T, the composite cam 110 is not completely ground. Namely, the composite cam 110 has an unground part F at the boundary between the first and second cams 112, 114 in the range of the common base circle portion C. In FIGS. 22 and 23, the unground part F is shown exaggerated in order to facilitate understanding. Specifically, the size of the unground part F is on the order of several micrometers.
If the composite cam 110 has the unground part F at the boundary between the first and second cams 112, 114 in the range of the common base circle portion C, the tappet gets over the unground part F when relatively moving between the first and second cams 112, 114. The operation of moving the tappet relative to the first and second cams 112, 114 is therefore not smoothly performed, which affects valve opening control. The grinding wheel T therefore need be trued frequently.
The problem that the composite cam 110 has the unground part F will be described specifically. As shown in FIGS. 22 and 23, the grinding wheel T has a greater axial width than the first cam 112 for low speeds and the second cam 114 for high speeds. The grinding wheel T becomes blunt at both ends Ta, Tb of its grinding surface as a workpiece, or a cam, is ground with the grinding wheel T. Namely, the grinding wheel T is worn faster at both ends Ta, Tb of its grinding surface than in the middle thereof, causing blunting of the ends Ta, Tb of the grinding wheel T.
As shown in FIG. 22, in the case of performing plunge grinding of the first cam 112 for low speeds, the grinding wheel T is positioned such that its right end Ta is aligned with the boundary between the first and second cams 112, 114. The left end Tb of the grinding wheel T therefore projects beyond the left side of the first cam 112. Blunting of the left end Tb of the grinding wheel T thus does not affect grinding of the first cam 112. However, blunting of the right end Ta of the grinding wheel T affects grinding of the first cam 112 side of the boundary between the first and second cams 112, 114, leaving an unground part F. The black part in FIG. 22 shows the unground part F. In FIGS. 22 and 23, grinding allowances of the first and second cams 112, 114, which are shown by phantom lines, are also shown exaggerated in order to facilitate understanding.
As shown in FIG. 23, after grinding of the first cam 112 is finished, the grinding wheel T is relatively moved to the position of the second cam 114 to perform plunge grinding of the second cam 114. In this plunge grinding, the grinding wheel T is positioned such that its left end Tb is aligned with the boundary between the first and second cams 112, 114. The right end Ta of the grinding wheel T therefore projects beyond the right side of the second cam 114. Blunting of the right end Ta of the grinding wheel T thus does not affect grinding of the second cam 114. However, blunting of the left end Tb of the grinding wheel T affects grinding of the second cam 114 side of the boundary between the first and second cams 112, 114, leaving an unground part F. This unground part F together with the unground part F of the first cam 112 in FIG. 22, both shown black in FIG. 23, remains at the boundary between the first and second cams 112, 114.